Enhance validate-system command with system output validation + introduce partitions directory

Fixing #12 (closed)

The validate-system now validates three things:

• That the container_outputs directory contains at least one sub-directory, containing at least one *output.json system output,
• That each reported system output is different from the baseline's,
• The directory name starts with a dataset name

These three checks only cause a failure if a --strict flag is provided. This design is picked so it is backward-compatible with the previous format of the container_output directory

Also, the index files and the activity index files have been moved in a new directory called partitions. The container_outputs can now contain the outputs only.